Strategy Design Pattern in Java – Example Tutorial

Strategy pattern is one of the behavioral design pattern. Strategy pattern is used when we have multiple algorithm for a specific task and client decides the actual implementation to be used at runtime.

Strategy pattern is also known as Policy Pattern. We defines multiple algorithms and let client application pass the algorithm to be used as a parameter. One of the best example of this pattern is Collections.sort() method that takes Comparator parameter. Based on the different implementations of Comparator interfaces, the Objects are getting sorted in different ways, check this post for sorting objects in java using Java Comparable and Comparator.

For our example, we will try to implement a simple Shopping Cart where we have two payment strategies – using Credit Card or using PayPal.

First of all we will create the interface for our strategy, in our case to pay the amount passed as argument.

Strategy Pattern Class Diagram

Important Points

We could have used composition to create instance variable for strategies but we should avoid that as we want the specific strategy to be applied for a particular task, same is followed in Collections.sort() and Arrays.sort() method that take comparator as argument.

Strategy Pattern is very similar to State Pattern. One of the difference is that Context contains state as instance variable and there can be multiple tasks whose implementation can be dependent on the state whereas in strategy pattern strategy is passed as argument to the method and context object doesn’t have any variable to store it.

Strategy pattern is useful when we have multiple algorithms for specific task and we want our application to be flexible to chose any of the algorithm at runtime for specific task.

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